What decides a fronthaul link
A 5G fronthaul link carries the radio signal between the distributed unit and the radio unit. Inside a shelter the first few hundred metres are copper; everything beyond that is optical, and the module at each end sets the reach, the temperature range and the timing margin of the whole site. Ports are mostly 25G SFP28 today, but the part number still depends on four things: link distance, fibre count, where the module physically sits, and the coding the host expects.
This guide covers the options that fit common fronthaul layouts, from a hop inside one shelter to a remote radio unit reached on a single strand.
Why fronthaul moved to 25G
Massive MIMO and 100 MHz carriers push CPRI and eCPRI line rates past 10G per port. 25G SFP28 puts one lane on one port, maps eCPRI without extra aggregation, and is the default cage on new distributed unit and radio unit ports. 10G SFP+ stays in service wherever a legacy CPRI link at 9.8G is still running, so a fronthaul spare kit normally carries both rates.
Recommended optics for 5G fronthaul
| Module | Rate | Media / wavelength | Reach | Typical fronthaul use |
|---|
| 25G SFP28 SR | 25G | MMF, 850 nm | 70 to 100 m | Indoor link between units in the same hall or rack row |
| 25G SFP28 LR | 25G | SMF, 1310 nm | 10 km | Standard outdoor fronthaul across a site |
| 25G SFP28 ER | 25G | SMF, 1310 nm | 30 to 40 km | Remote radio unit, long span or sparse site layout |
| 25G SFP28 BiDi | 25G | SMF, 1270 / 1330 nm pair | 10 to 20 km | Single-strand plant where fibre count is the limit |
| 25G SFP28 CWDM / DWDM | 25G | SMF, CWDM or DWDM channels | 10 to 20 km | Several radio units sharing one fibre pair through a passive mux |
| 10G SFP+ LR / ER | 10G | SMF, 1310 nm | 10 to 40 km | Legacy CPRI 9.8G fronthaul still in service |
The reach column lists the standard classes used in fronthaul planning; the working limit is the measured link budget, and the part is confirmed against the host card. The full rate family is on the 25G SFP28 transceiver page.
Industrial temperature is not optional outdoors
A pole-mounted radio unit sees the full outdoor range. Commercial grade optics rated 0 to 70 C are the most common cause of summer fronthaul dropouts: the module drifts, the link flaps, and the fault is reported at the radio rather than at the optic. Specify the industrial grade for any unit that is not in a conditioned shelter, and state the grade in the request so it is not substituted. Cabinet-mounted equipment sits somewhere between the two, but a roadside cabinet in summer runs well above ambient and is normally treated as outdoor.
Timing, sync and latency on the fronthaul
Fronthaul carries PTP (IEEE 1588v2) and SyncE alongside the user plane, and the optical layer has to pass that timing without adding jitter. Two practical rules follow. Keep the FEC profile and the latency class the same along a chain, because mixing generations on one path changes the delay budget and makes faults harder to isolate. And on a link close to its reach limit, a longer-reach part is a better answer than an added element, since every extra element adds delay and one more point of failure.
When fibre is the constraint: BiDi and WDM
BiDi transmits on 1270 nm in one direction and 1330 nm in the other over a single strand, so it halves the fibre requirement at the same reach class. Where several radio units share one route, CWDM or DWDM modules plus a passive mux put multiple links on one fibre pair. Both are common on dense urban sites where new fibre is expensive or slow to obtain, and both need the far end matched as a pair. See BiDi transceivers and WDM transceivers for the channel and pairing options.
Compatibility and coding
Radio and transport hosts check the coding of the optical module before the port comes up. Modules supplied for a branded host are compatible parts coded for the platform you name, tested against the port, and are not manufactured by the brand of the host, with no claim of relationship to that brand. Send the host model with the order so the coding matches at first insert.
Platforms seen in this layer include Nokia, Huawei, ZTE and Samsung on the radio side, and Cisco, Mellanox, Arista and Juniper on the transport and switch side.
How to choose, in order
- Distance first: indoor hop SR, site span LR, remote radio unit ER.
- Fibre available: dual strand LR or ER, single strand BiDi, shared route CWDM or DWDM.
- Temperature: conditioned shelter commercial grade, pole or cabinet industrial grade.
- Timing: confirm the FEC and latency requirement with the radio vendor.
- Coding: match the host, and name the host model in the request.
- Spares: keep at least one spare per link type on site, because a failed optic takes the cell down with it.
Frequently asked questions
Is 10G SFP+ still used in 5G fronthaul?
Yes, wherever a legacy CPRI link at 9.8G is still in service. New builds use 25G SFP28, and both rates are usually kept as spares.
Can commercial temperature optics be used on a pole-mounted radio unit?
No. Use the industrial grade and state it in the order. Commercial parts are intended for conditioned rooms, and summer dropouts on outdoor sites usually trace back to the temperature grade.
BiDi or dual-strand LR for a 10 km fronthaul link?
Both cover 10 km. BiDi is chosen when only one strand is free; a dual-strand pair is used when it exists and spare capacity matters more than fibre savings.
Can fronthaul run over CWDM or DWDM?
Yes, with CWDM or DWDM modules and a passive mux, and it is the usual answer when several radio units share one route. Both ends of a channel have to be ordered as a matched pair.
Will these modules work with my radio vendor?
They are compatible parts coded for the host you name and are not manufactured by that brand. Send the host model, the link distance and the temperature range and the coded part is confirmed before shipment.
Send the host model, the link distance and the temperature range, and we confirm the coded part, the reach and the lead time. Related reading: 100G QSFP28 and 400G QSFP-DD and OSFP families for the backhaul and transport layers, and the full list of optics selection guides.
